The present invention relates to nondispersive, infrared gas analyzers.
Gas analyzers of the type to which the invention pertains are, for example, constructed as two-beam or two-beam-path instruments, in which a reference beam and a measuring beam of infrared radiation is derived from a common source. A chopper interrupts the two beams periodically to modulate them in an alternating, 180-degree out of phase fashion. One or both of the two beams are attenuated by the sample or measuring gas. In some instances, a reference gas is used in one of the beam paths. In either case, the attenuated beams are detected by radiation detectors which, for example, include cells containing the same type of gas whose concentration is to be detected in the measuring or sample gas. The two detection cells are connected to opposite sides of a differential pressure measuring chamber, being partitioned by a flexible membrane which serves as one electrode of a capacitor. The capacitor is an electrical pickup from which an electrical signal can be derived, being indicative of the concentration of the component of interest in the measuring gas. See, for example, U.S. Pat. Nos. 4,156,812, 3,970,387, 3,937,962, and 3,925,667. Gas analysis on the basis of measuring in some fashion the effect of absorption of radiation by the component of interest depends on frequency selectively by that component. If the sample and measuring gas includes another component whose absorption bands overlap, to at least some extent, the absorption band of the component to be detected, the measuring result will not be accurate and will, in fact, be the more erroneous, the larger the relative content of that other, interfering, component in the measuring gas. This phenomenon is called cross-sensitivity of the analyzer. Reducing cross-sensitivity means, reducing the effect these interfering components have on the measuring result. In other words, a reduction in cross-sensitivity increases the selectivity of the analyzer and its sensitivity with regard to the component of interest; the effective measuring range is enlarged toward the lower end.
Cross-sensitivity has been reduced on the basis of two approaches, also called positive and negative filtering (see, for example, Chemie-Ingenieur-Technik, Vol. 33, 1961, No. 6, pp. 428, 429). Positive filtering, for example, uses a cell filled with measuring gas and placed in the path of one beam, while a cell filled with a nonabsorbing gas is placed in the path of the other beam. Negative filtering describes a situation, in which the two beam paths include cells of similar length, both cells being flown through by the measuring and sample gas, and the one path includes additionally a filter cell filled with gas of the type to be detected. It was found that negative filtering reduces cross-sensitivity to a greater extent than positive filtering.